1. Field of the Invention
The present invention relates to the operation, in particular the control or regulation, of a generator in a recuperation system of a motor vehicle.
2. Description of the Related Art
In the interest of reducing CO2 discharge, and in light of constantly rising fuel prices, recuperation systems for reducing the fuel consumption and thus the CO2 emissions of motor vehicles are becoming increasingly significant.
The motor vehicle's electrical system has a considerable influence on fuel consumption and thus on the CO2 emissions of the motor vehicle. An additional electrical power output of 100 W results, for example, in additional consumption of about 0.1 1/100 km in the New European Driving Cycle (NEDC). A 500 W vehicle electrical system load in this case signifies an additional 0.5 1/100 km of consumption.
If the kinetic energy released upon braking, or the potential energy released when traveling downhill, is used to power the vehicle's electrical system, the proportional fuel consumption, and thus CO2 emission, can thereby be reduced. A further potential for reducing fuel consumption results when the electrical machine of the motor vehicle is also utilized in motor mode and can thus contribute to the drive torque of the vehicle (boost). The resulting shift in the operating point of the internal combustion engine of the motor vehicle allows additional fuel savings to be achieved.
In order to be able to utilize braking energy, a suitable generator is necessary together with a suitable energy reservoir such as, for example, a lithium ion battery or a double layer capacitor, as well as a suitable regulation strategy.
Conventional generators of the motor vehicle are voltage-regulated. This means that a target voltage is specified to the regulator or generator regulator, and is controlled by the generator regulator within a permissible tolerance range. FIG. 10 shows in this regard a schematic block diagram of a generator unit 1001. Generator unit 1001 has a generator 1003, a generator regulator 1005, and an evaluation apparatus 1007. Generator unit 1001 is coupled to electrical system 1009 of the motor vehicle.
Regulation of the excitation field by way of generator regulator 1005 allows the generator voltage to be held constant irrespective of the electrical system load, rotation speed, and temperature. The generator current, and thus generator torque, occurring at a target voltage depend here not only on the electrical system load but also on the state of the motor vehicle battery.
FIG. 11 furthermore shows a schematic block diagram of a recuperation system 1101. Recuperation system 1101 of FIG. 11 is, in particular, a recuperation-capable vehicle electrical system that is suitable, for example, for two voltage levels. A first voltage level is, for example, 14 volts, whereas a second voltage level is, for example 42 volts. Recuperation system 1101 of FIG. 11 has a generator 1103, a starter 1105, a load group 1107, and an electrical energy reservoir 1109. Recuperation system 1101 further has a DC/DC converter 1111 that is connected between the two voltage levels. Recuperation system 1101 furthermore has a motor 1113, a first load group 1115, a second load group 1117, and a battery 1119. The units 1103, 1105, 1107, and 1109 are operated at 42 volts. Units 1113, 1115, 1117, and 1119, conversely, are operated at 14 volts. DC/DC converter 1111 converts from 42 volts to 14 volts. The key components for recuperation in FIG. 11 are generator 1103, which can also be embodied as a starter/generator and thus operated in motor mode, and power battery 1109.
When a specific torque and thus a specific mechanical power output is requested from the generator at a specific rotation speed, however, the electrical power output and thus, in particular, the generator current must be correspondingly adjusted. If torque regulation is additionally necessary, continuous power output regulation is then necessary in the context of any change in rotation speed.
With a conventional voltage regulation system this is possible only with a certain complexity. On the one hand, the generator current that occurs at a specific generator voltage depends on the battery state. In other words, appreciably different generator currents can occur for the same voltage but for a different battery charge state and battery temperature.
On the other hand, tolerances in the context of voltage regulation due to the relatively low internal resistance of power battery 1109 can result in appreciably different generator currents. A small voltage shift, for example a slight offset in the voltage measurement, can consequently lead to an appreciable current excursion and thus an appreciable deviation in the generator's power output.
In order to reduce the aforementioned dependence of the generator current on the generator voltage, current regulation can be carried out by cascade regulation, in which current regulation is overlaid on voltage regulation. If the current regulating function does not run on the generator regulator, however, sufficiently fast and accurate current regulation requires a correspondingly fast voltage definition. This requires a correspondingly fast and fail-safe communication interface. In addition, the corresponding control unit that contains the current regulation function must know the generator current. This control unit must moreover furnish sufficient resources, such as computational performance reserve and bus load, for such regulation.
Published European patent document EP 1 646 522 B1 presents a motor vehicle electrical system having a multi-voltage generator/motor, which encompasses an electrical machine, a regulator, and a pulse width modulated inverter, for electrically powering a first electrical sub-system having at least one first load in which a first nominal voltage (42 V) is present, and a second electrical sub-system having at least one second load in which a second nominal voltage (14 V) is present, and having a control unit, the motor vehicle electrical system having a DC/DC converter downstream from the multi-voltage generator/motor.
Published German patent application document DE 197 55 050 A1 describes a device for supplying energy in a vehicle electrical system for at least two similar electrical loads, in particular electrically actuatable vehicle brakes that are connected to at least two voltage reservoirs chargeable by a generator, the vehicle electrical system being a multi-voltage electrical system having at least two different voltages, and a decoupling element being located between the voltage reservoirs.